Authoring a Policy Pack

CrossGuard is a beta feature and is subject to breaking changes. The open source --policy-pack flag is free and available for all to use. A preview of CrossGuard is also available in the Pulumi Console, which enables you to enforce policies across an organization. To get access, submit a request here.

Install prerequisites.
- Install Pulumi
- Install Node.js version 8 or later

Verify your version of Pulumi.

$ pulumi version # should be v1.6.1 or later

Create a directory for your new Policy Pack, and change into it.
```
$ mkdir policypack && cd policypack
```
Run the pulumi policy new command. Since Policy as Code is a beta feature, you will need to set PULUMI_EXPERIMENTAL=true as an environment variable or simply pre-append it to your commands as shown.

macOS Linux Windows
On macOS, you can run export PULUMI_EXPERIMENTAL=true or simply prepend it to your commands as shown.
```
$ PULUMI_EXPERIMENTAL=true pulumi policy new aws-typescript
```
On Linux, you can run export PULUMI_EXPERIMENTAL=true or simply prepend it to your commands as shown.
```
$ PULUMI_EXPERIMENTAL=true pulumi policy new aws-typescript
```
On Windows, you must first set the environment variable before running the command.

Windows cmd.exe
```
set PULUMI_EXPERIMENTAL=true
pulumi policy new aws-typescript
```
Windows PowerShell
```
$env:PULUMI_EXPERIMENTAL = 'true'
pulumi policy new aws-typescript
```

Tweak the Policy Pack in the index.ts file as desired. The existing policy in the template (which is annotated below) mandates that an AWS S3 bucket not have public read or write permissions enabled. Each Policy must have a unique name, an enforcement level, and a validation function. Here we use validateTypedResource that allows us to validate S3 Bucket resources.

// Create a new Policy Pack.
new PolicyPack("policy-pack-typescript", {
    // Specify the Policies in the Policy Pack.
    policies: [{
        // The name for the Policy must be unique within the Pack.
        name: "s3-no-public-read",

        // The description should document what the Policy does and why it exists.
        description: "Prohibits setting the publicRead or publicReadWrite permission on AWS S3 buckets.",

        // The enforcement level can either be "advisory" or "mandatory". An "advisory" enforcement level
        // simply prints a warning for users, while a "mandatory" policy will block an update from proceeding.
        enforcementLevel: "mandatory",

        // The validateTypedResource function allows you to filter resources. In this case, the rule only
        // applies to S3 buckets and reports a violation if the acl is "public-read" or "public-read-write".
        validateResource: validateTypedResource(aws.s3.Bucket, (bucket, args, reportViolation) => {
            if (bucket.acl === "public-read" || bucket.acl === "public-read-write") {
                reportViolation(
                    "You cannot set public-read or public-read-write on an S3 bucket. " +
                    "Read more about ACLs here: https://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html");
            }
        }),
    }],
});

You can find more example Policy Packs in the examples repo. Policy Pack best practices documentation details the best practices for writing a Policy Pack.

Testing the Policy Pack Locally

Policy Packs can be tested on a user’s local workstation to facilitate rapid development and testing of policies. This removes the step of publishing and applying policy packs to the Pulumi Console and lets developers reference a policy pack on their local workstation.

Run npm install in the Policy Pack directory.
Use the --policy-pack flag with pulumi preview or pulumi up to specify the path to the directory containing your Policy Pack when previewing/updating a Pulumi project.

If you don’t have a Pulumi project readily available, you can create a new project for testing by running pulumi new aws-typescript in an empty directory. This AWS example will create an S3 bucket, which is perfect for testing our Policy.

In the Pulumi project’s directory run:

macOS Linux Windows
```
$ PULUMI_EXPERIMENTAL=true pulumi preview --policy-pack <path-to-policy-pack-directory>
```
```
$ PULUMI_EXPERIMENTAL=true pulumi preview --policy-pack <path-to-policy-pack-directory>
```
Windows cmd.exe
```
set PULUMI_EXPERIMENTAL=true
pulumi preview --policy-pack <path-to-policy-pack-directory>
```
Windows PowerShell
```
$env:PULUMI_EXPERIMENTAL = 'true'
pulumi preview --policy-pack <path-to-policy-pack-directory>
```
If the stack is in compliance, we expect the output to simply tell us which Policy Packs were run.
```
Previewing update (dev):

     Type                 Name          Plan
 +   pulumi:pulumi:Stack  test-dev  	create
 +   └─ aws:s3:Bucket     my-bucket     create

Resources:
    + 2 to create
```

We can then edit the stack code to specify the ACL to be public-read.

const bucket = new aws.s3.Bucket("my-bucket", {
    acl: "public-read",
});

We then run the pulumi preview command again and this time get an error message indicating we failed the preview because of a policy violation.

Previewing update (dev):

     Type                 Name          Plan       Info
 +   pulumi:pulumi:Stack  test-dev  	create     1 error
 +   └─ aws:s3:Bucket     my-bucket     create     1 error

Diagnostics:
  pulumi:pulumi:Stack (test-dev):
    error: preview failed

  aws:s3:Bucket (my-bucket):
    mandatory: [s3-no-public-read] Prohibits setting the publicRead or publicReadWrite permission on AWS S3 buckets.
    You cannot set public-read or public-read-write on an S3 bucket. Read more about ACLs here: [https://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html](https://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html)

Now that your Policy Pack is ready to go, let’s enforce the pack across your organization.

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